Induction heating consists in generating what are referred to as eddy currents in an electrically-conducting element by means of a magnetic field. The magnetic field is generated by an inductor, adapted to match the region of the item to be heated, through which an alternating current is passed. The alternating current is itself produced by a generator, which adapts the frequency and amplitude of the current to produce the desired heating.
When using induction heating for cooking purposes, the object to be heated is an electricity-conducting receptacle. Although the invention described below may be applied to non-magnetic materials of the copper or aluminum type, heating magnetic materials is more specifically of interest (low-carbon steels, cast irons, magnetic inoxidizable materials). In the applications sold to the general public, the receptacle is generally of a diameter of between 120 and 280 mm and is between 1 and 4 mm thick. These diameters may be as much as 450 mm and the thickness 10 mm in professional applications.
The formula of skin thickness divided by the relative magnetic permeability xcexcr and electric conductivity "sgr" of the recipient is applied:
xcex4={square root over (2/xcexcoxc2x7xcexcyxc2x7"sgr"xc2x7xcfx89)}
which gives the frequency:
f=xcfx89/2xcfx80
being in the order of 10-50 kHz, used to produce effective heating in ferritic bases of a minimum thickness of 1 mm. The supply voltage of 50 or 60 Hz distribution network is generally rectified and filtered; the excitation frequency is produced by means of a generator, generally by resonance. This generator is connected to a generally flat inductor (referred to as a xe2x80x9cpancakexe2x80x9d) placed facing the base of the pan to be heated underneath an electrical insulation material and acting as a support for a plate, usually glass ceramic.
One of the difficulties of this known system is that of being able to heat receptacles of different materials, shapes and diameters, which are not known in advance, uniformly and in the best adapted manner. The designer of such a product has to strike a compromise by opting for an inductor of a diameter somewhere between the smallest and the biggest receptacle likely to be heated, generally about 180 mm. As cooking appliances of this type are more widely sold, manufacturers are now starting to sell specialised hobs of different diameters selected for a specific type of receptacle: hobs of 140 mm for small receptacles, hobs of 220 mm for large receptacles and even hobs of 280 mm for very large receptacles. The bigger the hob is, the higher the power must be. Each hob is therefore supplied by a different generator, a solution which is not industrially or economically attractive. Having studied the inductor, it is possible, by altering the number of turns and the space between two successive turns, to provide an arrangement in which the complex impedance Z=R+jxc2x7Lxc2x7xcfx86 in inductors of different diameters under load is more or less identical, which means that the same generators can be used on hobs of different dimensions. The power rating which is then defined for the hob of the largest diameter is limited when applied to hobs of small diameters. This solution is not economically attractive either because it means using high-power generators to transmit low power.
Some manufacturers use a high-power generator to supply different hobs by incorporating an electromechanical switch. Although this solution allows the power to be regulated from one hob to another by varying the cyclical ratio, it is not satisfactory in terms of cooking and operating noise. It should be pointed out that the impedance (L, R) of an inductor increases with the number of turns. Conventional inductors are made up of a strand of several wires of a small section wound in a spiral, either in turns one around the other or, if the space between the turn must be variable, on a matrix. An inductor of a small diameter will therefore have a lower impedance than a larger inductor and the generator will transmit, as a first approximation, a higher power P=Rxc2x7I2 in the small inductors, which is the opposite of the desired objective.
Finally, in the conventional application where the inductor is a simple coil, the centre of the load which corresponds to the centre of the coil is relatively large and is not heated by induction which means that the heat distribution may not be acceptable unless the receptable has good heat diffusion properties.
A second difficulty of the known system is that of avoiding irradiation of a magnetic field in the immediate area around the inductor where the user is likely to be. In practice, the system must be able to work with small receptacles, even with receptacles positioned off-centre. If the ratio of the size of the inductor to the size of the receptacle is high, then coupling will be mediocre, affecting the energy output. The effect of this is to generate a large leakage field in the immediate vicinity of the inductor. In the past, the standards governing electromagnetic compatibility have made it compulsory to limit this field. More recently, there has been a further sharp cut in the permissible levels due to the possibility of the appliance user being exposed to these leakage fields and the potential hazards it might pose to health.
Adapting the size of the inductor system to suit the load as proposed in document EP-92 400 362.2 is one way of reducing the leakage field. However, this method is not sufficient, even assuming the system were adapted turn by turn.
Another known approach is to place inductors in pairs in phase and in phase opposition, these inductors being in series so that an identical current can be passed through them. This latter concept, which has long been known (document EP-86 17 273), if difficult to apply because if a simple inductor is divided into inductor pieces, the effect of mutual inductance between one turn and the other turns in its vicinity will be considerably reduced, which will lead to a drop in the impedance of the conductor. Consequently, the current remains high even though the designer has opted for a generator system suited to low impedance levels.
In addition, because the total current from the generator passes through the turns, they must have a large section, which presents an added difficulty in terms of fitting in adequate number of turns (ampere turns) underneath the load to be heated. Accordingly, it is necessary to try and reduce the section of the conductor and still allow the inductor to heat, it also being possible for the inductor to be cooled by the load (document FR-96 05 978). This solution is of interest but is difficult to implement and implies a significant increase to the temperature of the interface, cancelling out one of the interesting features of induction, heating, whereby a xe2x80x9ccoldxe2x80x9d medium can be used for cooking, and hence increasing the risk of accidents due to burning.
The objective of the present invention is to propose an inductor for cooking by induction heating that is simple in design and exhibits a high impedance so that it can be used in conjunction with other similar inductors to form an induction-heated cooking hob and in which combinations in parallel or in parallel series and in phase opposition will make it possible to produce a full range of hobs whilst keeping the level of magnetic disturbance to a very low level, regardless of the type of receptacle to be heated on the hob or more generally the heating surface.
To this end, the invention relates to an induction-heated cooking hob of the type defined above, characterised by:
at least one inductor made up of a large number of turns with at least one conductor,
the conductor is rectangular in section and of a small thickness, the large face of the section being parallel with the winding axis of the conductor forming the turns,
a layer of insulation of a minimum small thickness between adjacent turns,
the winding is substantially rectangular in shape when seen in a plan view.
This high-impedance inductor requiring a reduced amount of space can be supplied by a single low-power generator; it may also be used in conjunction with other pairs of generator inductors to form cooking surfaces of different sizes and powers.
The inductor is wound in a spiral by winding the conductor around a rectangular shape so as to cover a surface facing the receptacle to be heated. The successive turns are not necessarily all contiguous.
Generally speaking, the thickness of the insulator is small, i.e. the invention proposes that the thickness should be less than half the thickness of the conductor.
By virtue of one interesting feature which provides power whilst preserving a modular nature, the inductor is made by winding several independent conductors in parallel or in parallel series.
It may be of particular interest to wind several conductors insulated from one another simultaneously to form an inductor. These different conductors can then be placed in parallel which will increase the effective section of copper whilst maintaining skin and proximity effects in the copper, thereby reducing inductor losses. This is useful if the current in the inductor remains high in spite of the fact that the inductors are arranged in parallel, for example in professional cooking appliances where the power is high. However, the main interest of this simultaneous winding is not to further reduce the losses in the inductors but to be able to couple several generators on a same inductor by connecting them to different conductors of the inductor, in order to produce high-power cooking hobs operating on reduced-power generators.
For the purpose of the invention, the insulator is laid on the conductor before it is wound to form the inductor. This insulator may also be wound at the same time as the conductor is wound, particularly if the inductor comprises several coils wound at the same time and supplied by different generators, which may give rise to high differences in potential between the different coils. The wound insulator may even be capable of withstanding high temperatures or at least much higher than the varnishes used to provide electrical insulation for conductors, which are generally not capable or withstanding temperatures greater than 220xc2x0 C. A combination of insulators would be conceivable in situations where the pre-laid insulation provides minimum insulation between turns and the wound insulation provides enhanced insulation between different coils. Finally, another interesting feature is that the insulation contains a thermo-setting resin, which will allow the assembly to be set by raising the temperature.
In order to make a cooking hob, it is of particular advantage to combine several inductors in parallel or in series/parallel on a single generator.
Four inductors may be connected in parallel, each being in phase opposition relative to its immediate neighbour so that the magnetic flux from adjacent inductors builds up underneath the load. By taking this approach, an inductor can be produced which, at a first approximation (leaving aside the effects of mutual inductance between the inductors) has an impedance under load equal to xc2xc (4Z)=Z; this corresponds to the impedance of a conventional inductor but the rate of irradiation in the area immediately around the pan is very low because there are as many inductors in phase as there are inductors in phase opposition. The inductor is preferably positioned so that one face is parallel with the front of the cooking surface so that irradiation is minimised specifically in the region where the user is located.
One of the considerable advantages or arranging inductors in parallel is that the total current of the cooking hob is distributed between the inductors, i.e. I/4 in the example described above. Since losses in the inductor are a function of the square of the current P=Rcopperxc2x7I2, they are divided by 16, which means that it is possible to use a single conductor with a small section to make the elementary coils. If these four inductors are connected two in series in two in parallel, the current in the two branches connected in series, assumed to be identical, will be divided by two and hence the losses by four. It may be of interest to form couplings in parallel series if several coils are wound simultaneously since, if the inductor has N turns, each coil will make only N/2 turns and it may be difficult to obtain an impedance of 4xc3x97Z on one coil, whereas the final inductor will be capable of achieving an impedance Z if the 4 coils are placed in parallel. In this latter case, it will be sufficient to achieve the impedance Z because placing two coils in series will give an impedance 2xc3x97Z and placing the two double coils in parallel will return the impedance to the desired level Z.
This type of inductor minimises losses and is therefore very economical. In practice, an inductor must produce a given impedance under load which, as an initial approximation, is a function of the geometric characteristics of the turns, the coil and the number of turns (ampere turns). Having conductors very close together significantly increases the effect of mutual inductance in the turns and makes it possible to obtain very high impedance levels with a conductor of a reduced effective length. An inductor of this type will require 15 to 20% less copper than a known inductor whilst producing superior performance.
The heat distribution in a known inductor based on a simple coil is relatively mediocre since the currents induced are zero at the centre of the load centred on the inductor and are maximum on a level with the half radius of the indicator.
For the purpose of the invention, since the inductor is replaced by elementary inductors, the xe2x80x9ccoldxe2x80x9d zone and the xe2x80x9chotxe2x80x9d are distributed in smaller zones in which the temperature is easily distributed into the receptacle by heat conduction, thereby producing more uniform heating.
Dividing the current by about 4 (for 4 inductors) is an interesting approach if the current and hence the power of the hob is high. For very high power levels, the number of coils in parallel is increased, which also has the effect increasing the size of the heating zone; these high powers are dedicated to receptacles of large diameters.
For lower powers, there is no need to divide the current and two coils in parallel will suffice. However, there will be an increase in the leakage field at the sides of the cooking surface and it is therefore useful, as with the example of several simultaneously wound coils, to make each inductor unit by connecting two inductors of an impedance Z in series and in phase opposition, the final impedance being (2Z//2Z) and the electromagnetic leakage level at the side will again be very much reduced.
By virtue of one advantageous feature along the lines explained above, the adjacent inductors are connected to the high-frequency supply to generate additional magnetic flux underneath the load.
Along the same lines of thinking, the hob has an equal number of inductors for each of the two flux directions so that the magnetic radiation is compensated over a certain distance.
It is also of interest to combine the hob with one or more temperature sensors comprising:
heat conductive strips placed between at least two adjacent inductors,
these strips being connected to the actual sensor in order to transmit the temperature thereto.
If a hob has several inductors which may be used separately or in non-dedicated groups, i.e. adjacent inductors which may be combined relatively in any number, it is useful to indicate which inductors are being used and to this end the hob comprises:
an illuminated display means made up of illuminated segments mounted between two adjacent inductors,
a control circuit comprising a sensor detecting the presence of an object to be heated above an inductor so as to control the illuminated segments associated with this inductor.
The modular nature of hobs made using the inductors proposed by the invention is particularly useful if each inductor is supplied by an associated generator.
Using the invention, it is possible to make induction-heated cooker hobs or more generally heating surfaces with one or more hobs, each made up of one or more associated inductors.
Within the meaning of the present invention, the expression xe2x80x9cinduction-heated cooking hobxe2x80x9d is used indiscriminately to refer to a hob in the conventional meaning of the word, i.e. an area for heating a receptacle or a heating surface with several combined heating zones, of the type known from the prior art where a xe2x80x9cfrontxe2x80x9d hob and a xe2x80x9cbackxe2x80x9d hob are provided, one being low power and the other high power. Unlike the prior art, however, the front hob and the back hob may be made up of several modular inductors and in particular each may comprise 4 inductors as proposed by the invention.
In order to facilitate manufacture of the induction-heated cooker hobs and improve the inductors modular nature at the stage when they are made, it is also of particular advantage if an inductor is made so that it comprises:
a top surface formed by a protective, electrically insulating and optionally heat-insulating layer underneath which the inductor is placed,
a layer of material with a high magnetic permeability and low electric conductivity such as ferrite underneath the inductor in order to loop the magnetic circuit of the inductor,
an electromagnetic screen which may serve as a means of dissipating the heat underneath the layer of ferrite,
a printed circuit board bearing the components of the generator associated with the inductor, this board being mounted underneath the diffuser screen, the power components of the generator which need to be cooled being in thermal contact therewith,
the conductor supplying the inductor being directly connected to the printed circuit.
In an inductor or group of inductors of this type, the components of the printed circuit board are located on the top face, i.e. on the side of the diffuser combined with the ferrite layer and the inductor, whilst the bottom face of the printed circuit board serves solely as a means of connecting the different components or mounting the surface-mounted miniature components (SMC).
This being the case, it is of particular advantage if the conductors of the inductor cross through the printed circuit board and are wave-soldered at the same time as the connections for the printed circuit components, at the bottom face of the printed circuit.
The inductor proposed by the invention is made by winding one or more conductors. The winding process is advantageously achieved by winding the inductor starting from its outer turns. This will guarantee to produce the requisite shape and size of the coil, which is particularly important if coils are to be placed adjacent to one another. To this end, the outer turn is placed against a matrix of a desired shape and size, and the specific winding machine forms the coil by applying turns in succession against this outer turn, the tolerance of the coil being shifted to its centre without having any particular repercussions on the characteristics of the coil.
More specifically:
the conductors of the inductor are placed so that they are pushed through the printed circuit and are wave-welded at the same time as the connections for components of the printed circuit, at the bottom face of the printed circuit,
the conductor is wound forming the turns of the inductor to be produced starting from the outer turn which is made to the desired shape, and the successive turns are placed towards the interior of this outer turn,
a rectangular shape, in particular a square shape, is imparted to this outer turn,
several conductors are wound simultaneously to form an inductor comprising several conductors.